1. Field of the Invention
This invention relates to a control system for setting a clamping pressure of a continuously variable transmission which is mounted on a vehicle, neither too much nor too little, by learning the clamping pressure in every operating state of the vehicle or a prime mover, and by reflecting the learned result of in the control of the clamping pressure.
The disclosure of Japanese Patent Application No. 2004-041756 filed on Feb. 18, 2004 including specification, drawings and claims is incorporated herein by reference in its entirety.
2. Discussion of the Related Art
A belt type continuously variable transmission or a traction type continuously variable transmission transmits a torque by utilizing a frictional force between a belt and pulleys or a shearing force of traction oil between discs and rollers. Accordingly, a torque capacity of the continuously variable transmission of those kinds is set in accordance with the pressure acting on a portion where the transmission of the torque occurs.
The aforementioned pressure in the continuously variable transmission is called “clamping pressure”. When this clamping pressure is raised, the torque capacity can be increased to avoid a slip. On the contrary, there arises a disadvantage that a power is consumed more than necessary for establishing a high pressure or that the efficiency of power transmission is degraded. Therefore, the clamping pressure or applying pressure is generally set as low as possible within a range of no unintentional slip.
In a vehicle carrying the continuously variable transmission, for example, the mileage can be improved by controlling the speed of an engine with the continuously variable transmission. In order not to deteriorate this advantage, therefore, the clamping pressure is controlled to a level as low as possible within a range of no slip so that the power transmission efficiency in the continuously variable transmission may be improved as high as possible. Theoretically, the demand clamping pressure of the continuously variable transmission can be determined by using a structural parameter, such as a friction coefficient or an angle to clamp the belt by pulleys, as a variable, and by using a parameter representing an operating state such as an input torque or a running radius of the belt, as a constant. However, the variable such as the input torque as well as the constant such as the friction coefficient are not constant due to the individual difference, the aging or the estimated error. Therefore, it is difficult to determine an accurate value of the clamping pressure in advance. For this reason, the clamping pressure corresponding to the pressure in which the slip starts occurring (i.e., the slip limit pressure) or to the actual input torque, is measured or learned on the basis of the actual operating state or operative condition of the continuously variable transmission, and the measured or learned result is reflected on subsequent control.
One example is disclosed in JP-A-2001-12593 which discloses a transmission provided with a pair of conical discs and a driving member. In this transmission, the contacting force between the conical discs to clamp the driving member is varied to determine the slip limit, and is adjusted so as not to exceed the slip limit, in case the force to be transmitted, the speed, the transmission ratio or combination of those is substantially constant.
In the invention disclosed in JP-A-2001-12593, the contacting force is lowered within a range of no slip by controlling the contacting force on the basis of the detected slip limit. A characteristic field, which relates to the speed, torque, the speed change ratio and temperature, and which indicates the contacting force necessary for a specific slip, is so stored as to adjust the contacting force to correspond to the stored characteristic field.
The clamping pressure in the continuously variable transmission or the aforementioned contacting force is the pressure to set the torque capacity to transmit the inputted torque neither too much nor too little. Therefore, the learning of the clamping pressure or the contacting force is carried out in response to the operating state such as the torque or the speed, and the learned value is reflected on the control of the clamping pressure, under the operating state identical or similar to the operating state where the learning is carried out. In the invention disclosed in JP-A-2001-12593, moreover, the slip limit is determined by changing the contacting force under the operating state where a predetermined condition is satisfied. Therefore, the slip limit or the clamping pressure of various kinds of operating state cannot be determined or learned over a wide range unless the operating state of the continuously variable transmission or the vehicle having the continuously variable transmission changes widely. In other words, according to invention disclosed in JP-A-2001-12593, detection of the slip limit or the learning of the clamping pressure may be restricted.